In a heating, ventilating, and air conditioning (HVAC) system, air flow is typically controlled using resistors to slow down the flow of air at different points to obtain the proper air balance throughout a building. When one resistor is adjusted, the pressure level throughout the HVAC system will change; any change in the HVAC system pressure will affect the flow of air past every other resistor. Thus, adjusting a resistor at one point causes "cross-talk" with resistors at other points.
One of the most complex problems confronted by the HVAC industry is controlling air flow through process chambers, such as the clean rooms used in semiconductor integrated-circuit chip manufacturing, or the medical and biotechnology laboratories kept at below atmospheric pressure to prevent potentially dangerous microbes from blowing out of the laboratories. Some air exits the process chamber through process equipment and other work stations with fume hoods. A partial vacuum is usually required in such equipment in order to ensure that noxious fumes or dangerous microbes do not leak from the process equipment or fume hoods and thereby endanger personnel working nearby. It is frequently important that a constant partial vacuum be maintained in the process equipment in order to minimize defects in the integrated circuit chips being manufactured. In some process equipment it is important that only a small partial vacuum be maintained.
U.S. Pat. No. 3,053,272 to Babson shows two embodiments of a vacuum regulator, which uses a piston having a top face exposed to fluid pressure just upstream of the piston, and a bottom face exposed to the environment pressure. Fluid flows between the top and bottom faces of the piston. Babson shows only one regulator to control the flow from several lines. U.S. Pat. No. 4,732,190 to Polselli shows a shut-off valve using pistons having a structure similar to that of the Babson piston.
U.S. Pat. No. 3,766,933 to Nicholson, Jr., shows a vacuum release valve, which vents the fluid conduit to the atmosphere under certain conditions. U.S. Pat. No. 3,312,241 to Bryant shows a flow control device, which uses a split-airfoil structure to alter the impedance on the flow. U.S. Pat. No. 4,092,998 to Taplin shows several embodiments of an automatic pressure regulator having a rolling diaphragm. The position of the piston and the rolling diaphragm is determined by the force applied by a spring under tension, and the fluid pressures just upstream and just downstream of the constriction point.
U.S. Pat. No. 3,978,883 to Petersen shows a regulator with two independently movable elements located on opposite sides of the flow conduit. U.S. Pat. No. 4,873,873 to Day shows a metering system having two gates located on opposite sides of the flow conduit.
U.S. Pat. No. 4,250,915 to Rikuta shows several embodiments of flow regulating valves. In each of the embodiments the weight of piston tends to narrow the flow path or a portion of the flow path.